Includes abstract. / Includes bibliographical references. / This research investigates the feasibility of producing Ti-6Al-4V products by creating low cost BE sintered preforms and then subjecting these preforms to thermomechanical processing (TMP) in an attempt to both improve the relative density and refine the microstructure. The powders used were direct reduction titanium powder and elemental aluminium and vanadium powders.

Bibliography: leaves 106-113. / The abrasive wear resistance of ruthenium aluminide intermetallic and ferritic steel containing an iron-chrome sigma intermetallic phase have been investigated in this study. A medium carbon wear resistant steel (MCV) was used in the study to facilitate comparison between wear resistances in the materials of interest. Specimens of ruthenium aluminide, MCV and ferritic stainless steels containing a sigma phase were produced. The MCV steel was quenched and tempered to match the bulk hardness of the as-received ruthenium aluminide. Five different grades of thermomechanically worked ferritic steels specimens were heat treated to produce different volume fractions of sigma phase. The mechanical properties of the specimens were investigated by compression testing and microhardness measurements. Abrasion testing was carried out on a pin on belt abrasion apparatus. The surface response of the specimens to abrasive wear was characterised by optical and scanning electron microscopy. Microhardness of the specimens were measured with a digital microhardness machine. The wear resistance of ruthenium aluminide was found to be higher than all materials tested in this project. The wear resistance in some of the ferritic steels containing sigma phase was comparable to that of the wear resistant medium carbon steel. The surfaces of the specimens were shown to work-harden during the abrasion process. The corresponding work-hardening results showed that ruthenium aluminide had the highest work-hardening rate. It can be concluded that the work-hardening ability of the test materials correlates with their respective wear resistance properties.

This thesis investigates the influence of nitrogen concentration on the deformation behaviour of a modified AISI 200 type alloy. The Fe-18Cr-4Ni-7Mn base alloys contained a range of nitrogen contents from 0 to 0.27 wt%. The tensile behaviour was characterised by uniaxial tensile testing. The room temperature yield strength was shown to decrease with increasing nitrogen content for nitrogen contents less than 0.1 wt%. This decrease was attributed to the presence of secondary phases in the annealed state. An increase in yield strength was observed with further additions of nitrogen and this is probably due to solid solution strengthening and a Cottrell interaction. The variation of room temperature ultimate tensile strength showed no dependence on nitrogen content. Elevated temperature tensile tests (120DC) showed an increase in ultimate tensile strength and yield strength with nitrogen content suggesting that solid solution strengthening and a Cottrell interaction are occurring in this alloy range. Room temperature deformation was shown to induce a transformation from austenite to martensite throughout this alloy range, the degree of transformation decreasing with increasing nitrogen content. This transformation was shown to provide considerable strengthening and work hardening to the alloy. The low stacking fault energy (SFE) of this alloy range ensured that cross-slip was significantly inhibited during room temperature deformation and the deformation mode was observed to be planar glide. Nitrogen was shown to increase the SFE of this alloy range thus causing a decrease in the inhibition of cross-slip with increasing nitrogen content. Increasing the nitrogen content thus causes a decrease in strength if the contributions of a deformation-induced transformation, SFE variation and secondary phases are considered but causes an increased strengthening contribution due to solid solution strengthening and a Cottrell interaction. Hence as the nitrogen content increases different strengthening mechanisms are activated and no overall trend of room temperature ultimate tensile strength with nitrogen content is observed. The ductility and formability of the steels did not appear to show any distinct trends with changes in nitrogen content.

Includes bibliographical references (leaves 124-127). / Hydrogen was used as a temporary alloying element in CP Ti and Ti-6AI-4V. The microstructural evolution and phase transformations were monitored, before, during and after hydrogenation with in-situ dilatometric testing. Wrought CP Ti and Ti-6AI-4V specimens were pre-annealed and experienced four consecutive thermal cycles (Cycles 1-4) i.e. hydrogenation, post-hydrogenation, dehydrogenation and post-dehydrogenation, during dilatometric testing. The specimen in each thermal cycle was heated to 1000°C, heating rate 1°C/min (with an isothermal hold at 1000°C for three hours for hydrogenation and dehydrogenation cycles) and then cooled to room temperature at cooling rate of 1°C/min.

Includes bibliographical references. / The aim of this work was to investigate changes in the electronic structure of platinum as a result of alloying with vanadium, and the effects of these changes on O2 adsorption. This is important for the further development of hydrogen fuel cells, because the oxygen reduction reaction (ORR) presently requires O2 adsorption to occur on pure platinum, which is a prohibitively expensive material. A computational study has therefore been undertaken on alloying platinum (which reduces cost) with vanadium (for which there is plentiful experimental data) and the consequences for O2 adsorption. The first moment of the d-band of platinum alloy DOS was used to represent the d-band centre. The d-band centre of Pt-PDOS became lower as a result of hybridisation between platinum and vanadium. The d-band centre of a pure platinum surface with respect to the Fermi level is -1.99eV, but it is shifted to -3.23eV when vanadium atoms are added to the subsurface layer. The adsorption energies of O2 are sensitive to a combination of calculation parameters used. In this work, the calculations were executed using the CASTEP code. This is a plane wave pseudo potential code. The most stabilised geometry of an adsorbed molecule on pure Pt (111) was at the fcc site and had an adsorption energy of -1,91eV. The adsorption energy at the bridge site of Pt (111) is -1.81eV. When subsurface vanadium atoms were introduced, the equilibrium surface-molecule bond lengths increased. The adsorption energy at the fcc site shifted to -1.37eV, -1.43 for the bridge site and -1.45eV for the hcp site. It was concluded that the presence of vanadium atoms in the surface region destabilises an adsorbed oxygen molecule but a more detailed study is needed to show the effect of the solute atoms on the thermodynamics and kinetics of the whole oxygen reduction reaction chain.

Includes bibliographical references. / The work covered in this thesis provides a comprehensive discussion of the transformation behaviour of Type 304 metastable stainless steels with small' variations in alloy composition. The study focuses mainly on the austenite stability with respect to alloy composition, rate of deformation and temperature. To achieve these objectives, uniaxial tensile tests at 0.3 true strain were performed at low and high strain rates (10-3s-1 and 3 x 10-2s-1 respectively), in the temperature range of -60 to 55°C under isothermal testing conditions.

Includes bibliographical references. / The erosion behaviour of titanium aluminide intermetallic alloys has not been widely reported in the scientific literature and is part of the current international research effort aimed at exploiting these materials for turbine engine and automotive applications. In the present study titanium aluminides have been subjected to both solid particle erosion and cavitation erosion. The erosion rates have been measured and the damage mechanisms have been identified and discussed in terms of the microstructures and mechanical properties of the titanium aluminide alloys. This has been achieved with a variety of investigative techniques, including electron microscopy, mechanical testing and microstructural examination; and, where necessary, the erosion performance of other materials have been evaluated for comparison. In particle erosion, conducted with air blast rigs at room temperature and at elevated temperature, the titanium aluminide alloys exhibit a ductile mode of material removal, and their limited strain to fracture results in higher particle erosion rates than those for 304 stainless steel. Heat treatment to produce changes in microstructure and hardness does not significantly affect particle erosion performance, and elevated temperature tests reveal an increase in particle erosion rate with increasing temperature. In cavitation erosion, the titanium aluminide alloys exhibit a ductile mode of damage accumulation and material loss, and the rates of material loss are lower than those for other engineering materials such as 304 stainless steel and some hardmetal grades which are currently used in erosive environments. The mechanism of cavitation erosion of the Ti₃Al-based alloy involves the accumulation of strain in phase boundary regions and the preferential removal of the more brittle component of the microstructure. For the Ti₃Al-based alloy, cavitation erosion resistance increases with an increase in hardness produced by heat treatment. The TiAl-based alloys exhibit twinning during the initial stages of cavitation, which is characteristic of the high strain rate deformation of TiAl-based alloys, followed by substantial work hardening and preferential material loss from phase interfaces and twinned regions.

Includes bibliographical references. / A study has been made of the tribological behaviour of ultra high molecular weight polyethylene (UHMWPE) and ion implanted UHMWPE during water lubricated reciprocating sliding against differing stainless steel and Yttria Partially Stabilised Zirconia (YPSZ) counterfaces. A new laboratory test apparatus was designed, built and commissioned to facilitate this research. The new apparatus is capable of simulating the reciprocating wear of a sliding couple under diverse conditions of pressure, sliding speed and lubricant environment, as well as allowing the measurement of frictional forces 'encountered between the two surfaces in sliding contact. Variation in stainless steel counterface surface roughness resulted in three different types of wear behaviour.

Bibliography: p. 100-106. / This thesis examines the influence of austenite potential and hot roll finish temperature on the evolution of microstructure in the ferritic stainless steel grade AISI 430. In particular, it focuses on the influence of these variables on the hot band annealing behaviour of this steel. The material employed was obtained from laboratory and commercial heats. Two hot roll finish temperatures, viz. 600°C and 800°C for the commercial heats and two alloy compositions of austenite potential 11 and 61% for the laboratory heats were studied. Electron channelling contrast (ECC) obtained in scanning electron microscopy was used to follow the evolution of microstructure. Limited micro texture measurements were made using electron backscattered techniques. It was found that a low finish temperature produced a hot rolled microstructure that showed limited softening and a fully recrystallised microstructure after annealing while a high hot roll finish temperature produced a completely softened as-hot-rolled microstructure and only partial recrystallisation after annealing. A high austenite potential encouraged the ferrite phase to undergo extensive continuous recrystallisation during hot band annealing. However, the affinity for the precipitation of carbo-nitrides tended to play a role in slowing down the process. On the other hand, the ferrite phase deformed in the presence of a low austenite content mostly underwent extended recovery during hot band annealing. The softening here was affected by a low driving force. The end microstructures after annealing were however similar in both cases in as much as they consisted of elongated structures. The martensite phase was found to behave similarly regardless of the austenite content, where both recovery to produce subgrains and occasional recrystallisation occurred. During final recrystallisation after cold rolling, the high austenite potential coupled with a short hot band anneal time resulted in incomplete recrystallisation. This caused sharper alpha fibre texture components in the final sheet. A long hot band anneal time however resulted in sharper gamma-fibre texture components. In the case of low austenite potential, a long hot band anneal produced a random texture in the final texture. Ridging was observed in all cases but a high austenite content was found to lessen its severity. Also, it (ridging) was reduced by the random texture produced by long hot band annealing in the case where the austenite potential was low.

MAX phase ceramics are ternary ceramics with both metallic and ceramic properties. The existing backing materials in grinding wheels can be made of ceramics or metals. In these applications, ceramics have the disadvantage of low toughness, and most metals have the disadvantages of relatively high density and intolerance to some very high temperatures. The MAX phases have a combination of the main advantages of both metals and ceramics: they are soft and machinable yet also heat-tolerant, strong and lightweight. Cubic boron nitride (c-BN) is a widely used abrasive in grinding wheels, which is exceeded in hardness only by diamond. Composites of c-BN and selected MAX phases may result in materials of some interesting and useful properties for application in industry. Firstly MAX phases, Ti₃SiC₂; Ti₃AlC₂ and Ti₂AlC were synthesised, then reaction couples of MAX-cBN are made in order to investigate the best conditions for composite synthesis, and to analyse the interfacial phases which occur. Finally, the MAX-cBN composites were synthesised from the reaction couple studies. The following results were obtained: 1. Samples synthesised to obtain Ti₃AlC₂ were largely composed of the Ti₂AlC, and thus synthesis of the Ti₃AlC₂ MAX phase was deemed unsuccessful. 2. Nearly pure samples of Ti₂AlC and Ti₃SiC₂ were successfully synthesised with high densities, 99.16% and 98.21%, respectively, of the theoretical density. 3. Reaction couple studies revealed that the Ti₃SiC₂ /c-BN couple was successfully made at 1400°C, 10MPa pressure for 30 minutes, and Ti₂AlC/c-BN couple was successfully made at 1500°C, 10MPa pressure for 30 minutes. The interfacial phases characterised by XRD and SEM found here were TiN, TiC, TiB₂ and AlN for the latter and TiN, TiS₂ and TiB₂ for the former. 4. These conditions were used to successfully synthesise MAX/c-BN composites where both could react and still remain intact. The interfacial phases characterised by XRD and SEM found here were TiAl, TiC, TiB₂ and AlN for Ti₂AlC/c-BN and TiN, TiC, TiS₂ and TiB₂ for Ti₃SiC₂ /c-BN. From these results the following conclusion was drawn: Ti₂AlC and Ti₃SiC₂ are fully compatible with c-BN in order to synthesise a composite with notable properties such as the fracture toughness, suggested by the observed fracture mechanism seen from the fracture surface of these composites.